Evidence of the preferential involvement of mu receptors in analgesia using enkephalins highly selective for peripheral mu or delta receptors.

In order to study the preferential involvement of mu or delta receptors in the analgesic effects of enkephalins, several peptides which selectively interact with these two kinds of receptors in peripheral organs were synthesized. The inhibitory potency on the electrically stimulated mouse vas deferens (delta receptors) of the short peptide Tyr-D-Ala-Gly-NH-CH(CH3)CH2CH(CH3)2 (6) is 2100 times lower (IC50 = 1220 nM) than that of the longer and more hydrophilic peptide Tyr-D-Ser-Gly-Phe-Leu-Thr (10) (IC50 = 0.58 nM). In contrast, the IC50 values of all the synthesized compounds on the guinea pig ileum assay (mu receptors) are in the same range (100-360 nM). Likewise, their analgesic activities in mice, measured on the hot-plate test after intracerebroventricular injection, are similar. Therefore, the dissociation between antinociceptive properties in mice and potencies on the mouse vas deferens unambiguously reflects a preferential implication of mu receptors in analgesia. The possible involvement of brain delta receptors in behavioral effects is discussed.